On a GroupWise 8.0.1 system, you cannot set Mailbox Size Limits on the Client Options, Disk Space Mgmt tab any higher than 4 Gb, if your GroupWise user's client version is 7.x or below.
The 7.x client still only recognizes up to only 4 Gb. So, your over-the-top users (who have mailbox sizes higher than 4 Gb) have to have a custom levels set to a Mailbox size limit of 0 (zero), and a suggested Threshold setting of 100.
When you eventually get to upgrading the GroupWise clients to 8.0.1, then you can set Mailbox limits to sizes above the 4 Gb limitation of the client. The new limitation is 8 Tb.
Disclaimer: As with everything else at Cool Solutions, this content is definitely not supported by Novell (so don't even think of calling Support if you try something and it blows up).
It was contributed by a community member and is published "as is." It seems to have worked for at least one person, and might work for you. But please be sure to test, test, test before you do anything drastic with it.